Method of stripping rhodium plating



Patented Oct. 13, 1936 UNITED STATES PATENT OFFICE Karl Schumpelt,

Union,

N. J., assignor to Baker & Company, Inc., a. corporation of New Jersey No Drawing. Application mm, 1935, Qerial No. 17,390

9 Claims. (01. 75-101) The use of electrodeposited rhodium for protecting silverware, jewelry or silver plated articles against tarnishing and scratching, or for producing a pleasing white and lustrous finish on certain metals such as stainless steel, has become popular and extensive. However, it is desirable in some cases to remove the rhodium plating, especially from silverware and other articles formed of or plated with silver, for various reasons 10 Various attempts have been made to remove the rhodium plating, for example, electrolytically by placing the article as the anode in acid solution, but the rhodium is so resistant to the action of acids that the silver or other metal of which the article is formed is injured or destroyed by the chemicals that have been used, so that such methods are impracticable. Therefore, it has been the general practice to remove the rhodium by grinding the rhodium plating from the article.

This is a slow, laborious and expensive process, and great care and skill are necessary to prevent damage to the article. Therefore a prime object of my invention is to provide a novel and improved practical, easy and inexpensive method of remov- 'ingthe rhodium plating without the article.

Generally, in electrodepositing rhodium for the purpose above-mentioned, it has been found desirable to electrodeposit a layer of nickel on the article preliminary to applying the finish layer of rhodium so that the rhodium is deposited on the nickel and a composite coating of nickel and rhodium results, and I contemplate the removing or rhodium plating by simply immersing the article in a solution containing an acid which will dissolve the nickel but which will not materially attack the metal of which the article is formed, for example silver. Also I contemplate forming on the surface of the article during the treatment thereof in the acid, a coating that will protect the metal of the article from attack by the acid regardless of the length of time that the article may remain in the acid. My method obviates the use of electric current and is inexpensive and safe-in operation.

According to the present invention the method of stripping a composite coating formed of a plurality of layers of different metals from a silver or silver plated article, wherein the outer layer consists in or comprises one or more of the metals of the platinum group and the undercoating is formed of base metal, is characterized by treating said coated article with a solvent that will dissolve the base metal but will not attack the metal of the article or the outer coating of the layer. Thus, in accordance with the invention the method of stripping a composite coating of a plurality of layers of nickel or rhodium from silver or silver plated article wherein the nickel layer or layers constitute underplating for the rhodium, 5 comprises the immersion of the coated article in an acid bath which will dissolve the nickel but will not attack the silver.

More specifically describing the invention, especially for removing a composite coating of nickel 10 and rhodium from a silver or silver plated article, the solution I prefer to use consists essentially of phosphoric acid and water, although it is desirable to add a small quantity of aluminum sulphate to introduce the sulphate ion, and also to 1 add a small quantity of sodium chloride to form a coating on the silver to protect the silver against attack from the acid. The formula in detail is So far only inorganic acids which do not seriously attack silver have been specifically mentioned, although some organic acids which do 40 not attack silver, such as acetic acid, may be used. Also acetic acid may be combined with hydrochloric acid as follows:

Hydrochloric acid 500 cc. Acetic Mm 200 cc. 45 Water a 300; cc.

Sodium chlorid 30 grams In all of the formula, the proportions of the components are not critical and may be widely 5o varied. i

The method is carried out at a temperature preferably ranging from.30 degrees C. to degrees C., and the time required for completion of the method depends upon the amount of the 55 electrodeposited metal. The article being treated is permitted to remain in the solution until the nickel has been dissolved and the rhodium flakes off. The silver in the article is not damaged even where the article has engine turned surfaces, and in fact the silver is protected against injury by a coating for example of silver chloride even upon prolonged immersion of the article in the solution.

The sodium chloride may be omitted where hydrochloric acid is present; and aluminum sulphate may be omitted although its presence is desirable. Furthermore, other metal sulphates such as ferric sulphate may be substituted for the aluminum sulphate.

In all cases the acid does not attack or affect the rhodium, but I have found that the acid solution permeates the rhodium coating which is usually extremely thin and then dissolves the layer of nickel beneath the rhodium. The removal of the nickel of course frees the rhodium from both the article and the nickel whereby the rhodium can be easily recovered.

After the nickel and rhodium have been removed from the article, the surfaces of the article can be easily polished. In this connection it is interesting to note that where phosphoric acid is used as in the first and second above stated formula, the surface of the silver in the article after treatment according to my method is in substantially the same condition as it was before the surface was electroplated.

Having thus described my invention, what I claim is:

1. The method of stripping a composite coating of a plurality of layers of nickel and rhodium from a silver or silver plated article wherein the nickel layers or layer constitute underplating for the rhodium and said method consisting in immersing the coated article in a bath comprising a mixture of phosphoric acid and water wherein the major portion is phosphoric acid.

2. The method of stripping a composite coating of a plurality of layers of nickel and rhodium from a silver or silver plated article wherein the nickel layer or layers constitute underplating for the rhodium and said method consisting in immersing the coated article in a bath comprising a mixture of phosphoric acid and water wherein the major portion is phosphoric acid and a small quantity of a metal sulphate.

3. The method of stripping a composite coating of a plurality of layers of nickel and rhodium from a silver or silver plated article wherein the nickel layer or layers constitute underplating for the rhodium and said method consisting in immersing the coated article in a bath comprising a mixture of phosphoric acid and water wherein the major portion is phosphoric acid and a small a quantity of sodium chloride.

4. The method of stripping a composite coating of a plurality of layers of nickel and rhodium from a silver or silver plated article wherein the nickel layer or layers constitute underplating for the rhodium and said method consisting in immersing the coated article in a bath comprising a mixture of phosphoric acid and water wherein the major portion is phosphoric acid and a small quantity of each of aluminum sulphate and sodium chloride.

5. The method of stripping a composite coating of a plurality of layers of nickel and rhodium from a silver or silver plated article wherein the, nickel layer or layers constitute underplating for the rhodium and said method consisting in immersing the coated article in a bath comprising a mixture of phosphoric acid and water in the proportions of two-thirds of a liter of phosphoric acid and one third of a liter of water.

6. The method of stripping a composite coating of a plurality of layers of nickel and rhodium from a silver or silver plated article wherein the nickel layer or layers constitute underplating for the rhodium and said method consisting in immersing the coated article in a bath comprising a mixture of phosphoric acid and water in the proportions of two-thirds of a liter of phosphoric acid and one third of a liter of water about 56 grams of aluminum sulphate and about 56 grams of sodium chloride.

7 The method of stripping a composite coating of a pluralityof layers of nickel and rhodium from a silver or silver plated article wherein the nickel layer or layers constitute underplating for the rhodium, said method consisting, in immersing the coated article in a bath comprising phosphoric acid, water and hydrochloric acid in the proportions of one-third of a liter each.

8. The method 01' stripping a composite coating of a plurality of layers of nickel and rhodium from a silver or silver plated article wherein the nickel layer or layers constitute underplating for the rhodium, said method consisting in immersing the coated article in a bath comprising phosphoric acid, water and hydrochloric acid in the proportions of one-third of a liter each, about 26-58 grams of aluminum sulphate and about 26-58 grams of sodium chloride.

9. The method of stripping a composite coating of a plurality of layers of nickel and rhodium from a silver or silver plated article wherein the nickel layer or layers constitute underplating for the rhodium and said method consisting in immersing the coated article in a bath comprising a mixture of phosphoric acid and water wherein the major portion is phosphoric acid and a small quantity of an aluminum sulphate.

KARL SCHUMPELT. 

